Toxoplasma gondii, an obligate intracellular protozoan, causes serious complications from congenitally acquired infection or from reactivation of latent infection in immunosuppressed hosts. We will examine host-parasite interactions so as to better understand T. gondii 's pathogenesis. T. gondii is a purine auxotroph that grows within a parasitophorous vacuole surrounded-by host cell mitochondria. Several lines of evidence suggest that T. gondii may use host cell ATP to support its own metabolism. To test this, ATP produced by either host cell or parasite must be selectively manipulated without affecting the other. Then T. gondii's dependence on ATP from each source can be determined. Brown adipocyte (BA) mitochondria uniquely uncouple oxidative phosphorylation, producing heat without ATP. BA will be infected with T. gondii, then BA mitochondria will be uncoupled to assess the effect of diminished host cell ATP on intracellular tachyzoite metabolism. Alternatively, T. gondii 's mitochondrion will be selectively inhibited, by infecting mutant host cells that are resistant to mitochondrial inhibitors. If T. gondii can grow within these resistant mutants in spite of inhibitors that should impair its own mitochondria, it would suggest that host cell mitochondrial function, ie ATP production, is important for the growth of intracellular tachyzoites. These two unique experimental approaches may yield important insights into T. gondii's possible dependence on host cell ATP. We have discovered that catecholamines (CAT) affect the metabolism of extracellular tachyzoites. CAT agonists and antagonists will be examined for effects on tachyzoite glucose utilization, oxygen consumption, RNA synthesis, and rates of invasion and replication. We will determine if T. gondii possesses CAT receptors, and then quantify and characterize any that are found. Tachyzoite signal transduction mechanisms will be examined by measuring changes in adenylate cyclase activity and cyclic AMP levels in response to CAT. We will determine if tachyzoites are capable of synthesizing CAT de novo. Finally, host CAT levels will be decreased in animal models of acute and chronic T. gondii infection to determine the effects of CAT on T. gondii 's pathogenesis in vivo. A better understanding of the basic cell biology and interaction between T. gondii and host cells may lead to new approaches to the therapy of this increasingly important human pathogen. Additionally, study of unicellular eukaryotes may yield insights into the evolution of signal transduction pathways in higher vertebrates.